1. Field of the Invention
The present invention relates to a method of concentrating a homogeneous catalyst from a process stream which contains this homogeneous catalyst as constituent. To concentrate the catalyst, the process stream is passed over at least one membrane.
2. Description of the Background
Catalyst systems used in homogeneous catalysis generally have to be removed entirely or partly from the respective reaction mixture. Reasons for this can be product purity requirements or the recovery of the catalyst system as a material of value which can be, for example, recirculated directly or indirectly to the reaction.
Particularly when using transition metal complexes such as rhodium as catalyst or when using expensive ligands, separating off the catalyst system is an important process step because of the catalyst costs. Industrial processes which are carried out using transition metal complexes in a homogeneous phase are, for example, telomerizations, metatheses, hydrogenations or hydroformylations. Reactions using rhodium complexes are widespread industrially and associated with relatively high catalyst costs.
Rhodium complexes are used as catalyst in, for example, the industrial hydroformylation of olefins to form the aldehydes and/or alcohols having one more carbon atom. In particular, rhodium complexes having phosphine, phospite, phosphonite or phosphinite ligands are used as catalyst here.
The way in which the catalyst is separated off can have a significant influence on the economics of the overall process. The separation of the catalyst from the reaction mixture can in the simplest way be carried out exclusively by means of thermal separation processes, for example by separating off the reaction product and if appropriate starting material from the catalyst-containing reaction mixture by evaporation. The disadvantage of such processes is that the catalyst and/or the ligand can decompose during the distillation. The catalyst decomposition products in the distillation residude can frequently not be converted into an active catalyst system in the process. They therefore have to be discharged and be worked up in a complicated manner before recirculation to the process. This applies in particular to the work-up of hydroformylation mixtures which comprise rhodium complexes having ligands which complex the rhodium less strongly than phosphines as catalysts. In the distillation, these ligand complexes can decompose because of the lack of stabilization by carbon monoxide, which can lead to formation of rhodium clusters. The rhodium clusters cannot be converted into the active catalyst under hydroformylation conditions. In addition, partial decomposition of the ligands can occur during the distillation.
A potential way of separating off homogeneous catalyst systems under mild conditions is concentration of process streams containing homogeneous catalyst by means of membranes in a single-stage or multistage arrangement.
EP 0 781 166 describes the separation of dissolved rhodium-organophosphite complex catalyst and free ligand from a nonaqueous hydroformylation reaction mixture over a membrane to an extent of at least 90% by mass of the catalyst and the free ligand. Membrane polymers mentioned are Teflon, polydimethylsiloxane (PDMS), polyethylene, polyisobutadiene, polystyrene, polymethyl methacrylate, polyvinyl chloride, cellulose diacetate, polyvinylidene chloride and polyacrylonitrile. The separation of the high boilers from the catalyst system is not described.
EP 1 232 008 describes the separation of high boilers from a catalyst recycle stream by means of a membrane (PDMS). The recycle stream arises in the work-up by distillation of an output from a reaction catalyzed by a metal-organic catalyst. Here, starting materials and the primary products are distilled off and a high boiler mixture in which the catalyst system is dissolved remains as bottom product. This is recirculated to the reactor. Since small amounts of high boilers are formed in the process, part of the high boilers have to be separated off from the bottom product in order to keep the concentration of high boilers constant. In EP 1 232 008, the high boilers are separated off from the bottom product with addition of a diluent. The diluent is added in such an amount that the proportion of high boilers in the solution which is fed to the membrane is less than 50% by mass. The high boilers are separated off in the temperature range from to 50° C. and in the pressure range from 0.1 to 10 MPa. The addition of diluents is disadvantageous because the amount of material passed over the membrane is increased. Furthermore, part of the added diluent is separated off with the high boilers, thus incurring costs for the diluent or for the recovery therefrom.
DE 10 2005 046250 describes a method of separating off a metal-organic catalyst system. Here, the organic reaction output is, in a first step, separated into a retentate containing the major part of the catalyst system and a permeate comprising starting materials, primary products, high boilers and catalyst system. The retentate is recirculated directly to the reactor. The permeate is separated by distillation into an overhead product which contains mainly starting materials and primary reaction products and a bottom product containing the catalyst system dissolved in high boilers. This document also discloses, as optional work-up variant, separating off part of the high boilers by means of a membrane from the bottom product before it is recirculated to the reactor. Principle membrane materials which can be used are stated as being polydimethylsiloxane (PDMS), polyimide (PI), polyamide imide (PAI), acrylonitrile/glycidyl methacrylate (PANGMA), polyamide (PA), polyether ether ketone (PEEK), polymers having intrinsic microporosity (PIM) and hydrophobized ceramic membranes. However, no details are given for the high boiler removal by means of membranes, for example membrane types which can be used.
DE 10 2005 060784 A1 describes a method of recovering a stream enriched in a metal complex catalyst (>200 dalton). The reactor output is separated by distillation into a low-boiling stream and a higher-boiling bottom stream containing the catalyst. The bottom stream is separated by means of membrane in a permeate stream and a catalyst-enriched retentate stream which is entirely or partly recirculated to the reaction. Only ceramic membranes having a separation limit (MWCO) of over 500 dalton and polymer membranes having separation limits over 10 000 dalton are indicated. Nothing is said about the activity as an important measure for the quality of the retained catalyst system.
The catalyst mentioned by way of example has a molecular weight of about 12 000 dalton and is therefore more than an order of magnitude larger than the conventional catalysts used in industry. The disclosure of DE 10 2005 060784 A1 cannot be applied to industrially relevant catalyst systems having a smaller molecular weight.
In addition, the retention on which the separation limit is based and the system for which the separation limit was determined are not stated. The information given is usually based, depending on the manufacturer, on a retention of 90 or 95%. The separation limit does not serve as absolute limit but as qualitative aid for selecting a membrane for a specific separation problem (see Melin, Rautenbach: Membranverfahren, second edition, 2004, Springer-Verlag Berlin, Heidelberg). It is therefore questionable whether the separation limits indicated apply to the metal complex catalyst. The necessity of retaining the ligand is not mentioned.
The membranes listed are exclusively porous membranes whose separation limits are, as shown below, not suitable for the process-relevant catalyst system. For the membranes listed, apart from various ceramic membranes only polytetrafluoroethylene, polyvinylidene fluoride (PVDF), polysulfone, polyether sulfone, polydimethylsiloxane (PDMS), polyether ketone, polyamide and polyimide are mentioned as possible membrane materials.
US 2006/0246273 A1 discloses a novel polymer having intrinsic microporosity (PIM). Owing to the rigid spiro bond, this polymer or polymer mixtures based thereon have a large free volume within the polymer matrix. The polymer can potentially be used for the separation or enrichment of gas, vapor or liquid mixtures. Owing to the high free volume, the polymer can be particularly advantageously used as membrane material for gas separation. Fritsch et al. (J. Mem. Sci. 251, 263-269 (2005)) achieve comparatively high permeabilities.
Apart from gas separation, PIM-based membranes can potentially also be used for the separation of chiral molecules such as amino acids, for the separation of organics, e.g. alcohol, from aqueous systems, for isomer separation, in pharmacy and biotechnology for the separation of proteins or other thermally unstable components, in fermenters and bioreactors for introduction of gas and biomass removal and also for the removal of microorganisms from air and water.
Further potential use opportunities are water purification, the detection or removal of trace components or metal salts from air or water, the separation of liquid mixtures by means of pervaporation (e.g. in ethanol production) and gas/vapor separation (e.g. separation of organic vapors from gas) and the liquid-liquid separation of organic components or improving the yield in equilibrium reactions by selective product discharge. The isolation of dissolved solids such as homogeneous catalysts is not described. Furthermore, the membrane thicknesses of from 10 to 500 μm which are indicated are too thick for the desired separation task.
WO 2005/113121 A1 describes the production of composite membranes having a thin layer of a microporous material having a high intrinsic microporosity. Apart from production of the membrane, possible uses of the membrane, e.g. fluid separation and the separation of low molecular weight solids from fluids, are mentioned. Mention is also made of the separation of hydrogen and hydrocarbons, N2 or CO, the separation of CO2, H2O and H2S from natural gas, the separation of nitrogen and oxygen, the separation of VOCs (volatile organic compounds) and other lower hydrocarbons from air and other gases, the separation of traces of organic components from aqueous streams and also the separation of low molecular weight components and oligomers from fluids and especially from solvents.
A disadvantage of the methods known from the prior art is that the methods either cannot separate off the catalyst system under sufficiently mild conditions and therefore with retention of activity and/or the active catalyst systems cannot be retained to a sufficient extent. Particularly in the retention of an active catalyst system having a molar mass below 1500 g/mol, which is significantly lower than that of clustered catalyst species, there is the difficulty of separating components having a molar mass difference of less than 100 g/mol from one another using a membrane. At the same time, the membrane has to be sufficiently permeable for organic components to allow an economical process.
Since no satisfactory solution is known for the concentration of a catalyst system under mild conditions from process streams from the hydroformylation of higher olefins (C4 and higher), where the catalyst system comprises the metal complex catalyst and clusters thereof and also the free ligand, it is an object of the invention to develop a method which separates the process stream containing the catalyst system (e.g. reactor output or bottom product) into a stream enriched in the catalyst system and a stream depleted in the catalyst system and has a high degree of retention for the catalyst system, in particular for the metal component.
The technical object of the invention is therefore to provide a method of concentrating a homogeneous catalyst, in which the catalyst system can be concentrated or separated off while retaining its activity, with the method having to have a high degree of retention for the catalyst system, requiring no diluents for the recovery and also displaying no clustering of the metal component of the catalyst or decomposition of the catalyst complex.